Method For Producing A Hollow Body By Cold Spraying And Mould Core Suitable For Carrying Out Said Method

ABSTRACT

A method is disclosed for producing a hollow body by cold spraying. A mold core is used for producing the hollow body, the mold core being prepared for the production of the hollow body by cold spraying in a suitable manner by means of an auxiliary layer. The auxiliary layer may include or consist of a metallic material and therefore forms a suitable surface to which the particles processed by cold spraying remain adhered to form the hollow body. The mold core can therefore be produced from an inexpensive material such as sand or wood, although said materials are in principle have limited suitability for depositing metals by cold spraying. The auxiliary layer can be applied to the mold core as a foil, or be produced on the mold core by cold-spraying low-melting and/or soft materials, for example.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a U.S. National Stage application of International Application No. PCT/EP2015/055611 filed Mar. 18, 2015, which designates the United States of America, and claims priority to DE Application No. 10 2014 206 073.7 filed Mar. 31, 2014, the contents of which are hereby incorporated by reference in their entirety.

TECHNICAL FIELD

The invention relates to a method for producing a hollow body. In this method, the hollow body is produced by coating a mold core by cold spraying. In other words, the coat produced on the mold core forms the hollow body to be produced. After the production of this hollow body, the mold core is removed from the latter. Moreover, the invention relates to a mold core having a surface suitable as a substrate for cold spraying. The surface must be suitable for cold spraying to the extent that the particles that are accelerated to the surface of the mold core by means of the cold gas jet must adhere to it.

BACKGROUND

Cold spraying is a method known per se, in which particles provided for coating are accelerated, preferably to supersonic speed, by means of a convergent-divergent nozzle to ensure that they adhere to the surface to be coated by virtue of their kinetic energy impressed upon them. In this process, use is made of the kinetic energy of the particles, which leads to plastic deformation of said particles, wherein the coating particles are fused only at the surface thereof upon impact. This method is referred to as cold spraying, in comparison with other thermal spraying methods, because it is carried out at relatively low temperatures, at which the coating particles remain substantially solid. For cold spraying, which can also be referred to as kinetic spraying, use is preferably made of a cold spraying system which has a gas heating device for heating a gas. Connected to the gas heating device is a stagnation chamber, which is connected on the outlet side to the convergent-divergent nozzle, preferably a Laval nozzle. Convergent-divergent nozzles have a tapering segment and a widening segment, which are connected by a nozzle throat. On the outlet side, the convergent-divergent nozzle produces a powder jet in the form of a gas stream containing particles traveling at high speed, preferably supersonic speed.

A method for producing a hollow body of the type stated at the outset is known from DE 10 2010 060 362 A1. Accordingly, cold spraying can be used to produce a tube on a cylindrical mold. In this case, the cold gas jet is angled relative to the surface of the cylinder to such an extent that the particles primarily adhere to the tube that is in production. The cylindrical mold core can therefore be removed from the tube after the production of the latter. This is possible because of the typical geometry of tubes which are free from internal undercuts, thus allowing the cylindrical former to slide along the inner walls of the tube. However, it is not possible to produce hollow bodies with more complex geometries in this way.

SUMMARY

One embodiment provides a method for producing a hollow body, in which the hollow body is produced by coating a mold core by cold spraying, and the mold core is removed from the hollow body after the production thereof, wherein the mold core is provided with an auxiliary layer before the production of the hollow body, wherein the material of the auxiliary layer is metallic or contains at least predominantly metallic components, and the auxiliary layer is removed from the hollow body after the removal of the mold core from the mold.

In one embodiment, the material of the auxiliary layer differs in composition from the material of the hollow body.

In one embodiment, the auxiliary layer is formed by a metal foil, in particular an aluminum foil.

In one embodiment, the metal foil is bonded adhesively onto the mold core.

In one embodiment, the at least one auxiliary layer is produced as a starting coat on the former by cold spraying, in particular by low-pressure gas dynamic spraying, of a metallic material.

In one embodiment, metallic materials having a ductile material behavior, in particular one or more of the metals or metal alloys based on the metals zinc, tin, lead, aluminum, copper, silver and gold, are deposited.

In one embodiment, the starting coat is of multilayer design, wherein, in the production sequence, the auxiliary layers are produced using increasingly harder and/or higher-melting metallic materials.

In one embodiment, the starting coat is produced in two layers comprising a base layer lying on the mold core and composed of one of the metals or metal alloy based on the metals zinc, tin and lead, and comprising a top layer following thereon and composed of one of the metals or metal alloy based on the metals zinc, aluminum, copper, silver and gold.

In one embodiment, the mold core is produced from wood, plastic, metal or bonded sand.

Another embodiment provides a mold core having a surface suitable as a substrate for cold spraying, wherein the surface is formed by at least one auxiliary layer composed of a metallic material, which forms a starting coat on the material of the mold core.

In one embodiment, said core consists of bonded sand, wood, metal or plastic.

BRIEF DESCRIPTION OF THE DRAWINGS

Examples aspects and embodiments of the invention are described below with reference to the drawings, in which:

FIG. 1 shows an illustrative embodiment of the method according to the invention, which is performed in a cold spraying system,

FIGS. 2 and 3 show illustrative embodiments of the mold core in section, according to one embodiment, and

FIG. 4 shows the detail IV as illustrated in FIG. 3.

DETAILED DESCRIPTION

Embodiments of the invention provide a method for producing a hollow body by means of cold spraying, by means of which it is possible to produce even hollow bodies of complex geometry. Other embodiments provide a mold core, or “former,” which can be used in such method.

In some embodiments of the method, the mold core is provided with an auxiliary layer before the production of the hollow body. The material of this auxiliary layer may be metallic or contains at least predominantly metallic components. This means that the auxiliary layer can provide a metallic matrix into which nonmetallic inclusions can be embedded. Such a layer nevertheless behaves substantially like a metal. The inclusions can be particles of a dry lubricant, for example, in order to facilitate removal of a core from the mold. The material of the auxiliary layer preferably has a different composition from the material from which the hollow body is to be produced. This advantageously allows optimum adaptation of the material which forms the surface for coating with the material of the hollow body to the requirements of a substrate for cold spraying. On the one hand, this material must be sufficiently ductile to ensure that the particles from the cold gas jet adhere to the surface of the mold core. Moreover, this material must be sufficiently temperature-stable, and, under some circumstances, allowance must be made for preheating the gas used to form the cold gas jet. In this process, the temperatures reached in the impact flow that forms ahead of the mold core can be approximately the same as those of the carrier gas in the stagnation chamber arranged upstream of the cold spraying nozzle. One other necessary property of the surface of the mold core is resistance to the erosive effect of the impinging particles. If this surface is mechanically too unstable, it does not offer sufficient resistance to the impinging particles of the cold gas jet, and therefore there would be no mechanical bonding of the particles to the surface and, as a consequence, the mold core would be destroyed.

If these properties of the metallic auxiliary layer are met, the material of the mold core chosen can advantageously be largely independent of the requirements of cold spraying. As a result, mold core materials, the mechanical stability of which would not normally be adequate for application of cold spraying, are also available for the disclosed method. However, it is precisely these materials which, on the one hand, are inexpensive to use and, on the other hand, are simple to remove from the finally produced molding. According to some embodiments, sand, wood, metal or plastic can be used for the mold core. Sand cores have the advantage that they can be produced at low cost as lost cores and can also be removed easily from the cavity of the cavity structure by dissolution of the bond between the individual sand grains. Wood forms a low-cost material which can be processed easily, especially in the case of very small batches, in order to produce cores of the required geometry. Metal is suitable especially for producing cores for multiple uses. The wear of said cores is advantageously low. Moreover, these can be manufactured with high dimensional accuracy. The surface of these metal cores is then protected from wear by the auxiliary layer. Plastic cores also have the advantage of simple production and a low-cost material, which can also be cast, for example.

In some embodiments, a mold core which can be produced, in particular, from the materials mentioned is then provided with an auxiliary layer of a metallic material, wherein this layer is involved in the formation of the starting coat on the material of the mold core or forms said coat alone, wherein the starting coat makes available the surface for subsequent cold spraying of the cavity structure.

As used herein, hollow bodies should be taken in the widest sense to mean all structures which have a concave inner surface and a convex outer surface. The concave inner surface can thus be supported by a former during production, while it is accessible to the cold gas jet from the convex side. Thus, for example, a bowl-shaped component would likewise qualify as a hollow body in the sense according to the invention, wherein the bowl-shaped depression would form the cavity with a correspondingly wide opening. Conventional hollow bodies would be, for example, housings, which can have a small opening in comparison with the cavity formed. Of course, the hollow bodies do not necessarily have to have exclusively convex outer walls either. There may also be concave areas on the outside. In one embodiment, the auxiliary layer is formed by a metal foil, in particular an aluminum foil. In this case, the metal foil is placed on the mold core and thus forms the metallic surface on which the hollow body can be deposited by cold spraying. Here, aluminum represents a very low-cost variant, wherein this material is, on the one hand, sufficiently ductile to ensure that the sprayed particles adhere. On the other hand, this metal is mechanically sufficiently stable to protect the former from erosion by the cold gas jet. On formers made of wood, for example, an aluminum foil with a thickness of 0.1 mm is sufficient to enable metallic materials to be deposited. For example, it has even been possible to deposit a hollow body composed of a titanium alloy thereon.

The thickness of the auxiliary layer must be chosen according to the hardness and sensitivity of the material of the mold core. If sand cores are used, for example, the coating thicknesses of the auxiliary layer are somewhat greater owing to the need for greater protection. In the case of lesser thicknesses of the auxiliary layer, it must be taken into account that this layer is deformed plastically owing to the impact of the particles of the cold gas jet. However, the plastic deformation must not lead to complete destruction of the auxiliary layer since the remaining mold core would then no longer be protected. If the auxiliary layer is embodied as a metal foil, it can advantageously be bonded adhesively onto the mold core. On the one hand, this avoids slippage of the foil during coating, especially at angles other than 90° between the cold gas jet and the surface. Moreover, adhesive bonding facilitates the application of the foil to the mold core, especially in the case of complex mold core geometries.

According to another embodiment, the at least one auxiliary layer may be produced as a starting coat on the former by cold spraying of a metallic material. Here, a metallic material is chosen deliberately, something that may be regarded as unproblematic as regards coat formation on the former in comparison with the material which is provided for the hollow body. In other words, the metallic material, which, in particular, can be formed by a very ductile material, remains on the former without destroying the latter. If the starting coat is applied in sufficient depth to the former, it then offers sufficient resistance during the deposition of the material of the hollow body. The thickness information given in relation to the embodiment as a foil applies in corresponding fashion to the thickness of the starting coat.

The production of the starting coat by means of cold spraying furthermore has the advantage that the starting coat can be built up in several auxiliary layers. In this way, it is possible to deposit metallic materials in succession, wherein, in the production sequence for the auxiliary layers, the procedure involves working with increasingly harder and/or higher-melting metallic materials. This means that the auxiliary layer, which is produced directly on the mold core, can be selected to ensure that the mold core is subject to as little mechanical stress as possible. This is the case especially with very low-melting and/or very ductile materials. Zinc, tin and lead, in particular, are used here. The following layers can then be produced from other metals, wherein zinc, aluminum, copper, silver and gold can preferably be used. In the case of the noble metals, it should be noted that these are very expensive to procure. However, these can assume special tasks in the hollow body, e.g. corrosion protection or an antimicrobial or catalytic action, potentially justifying the cost of their use. Instead of the abovementioned metals, it is, of course, also possible to use metal alloys which contain these metals as alloying components and have comparable mechanical properties.

In the choice of metals for the auxiliary layer which forms the mold core surface to be coated, it should be taken into account whether the requirement is more for thermal stability or for mechanical stability. Thermal stability is a higher priority in selection when the temperature of the cold gas jet is increased by preheating the carrier gas. At the same time, the processed particles are warmer in this case and therefore impose a lower mechanical stress on the surface of the mold core. Mechanical stability is a higher priority when the particles in the cold gas jet are themselves not very ductile and therefore cause higher mechanical stress in the mold core.

“Low-pressure gas dynamic spraying” (LPGDS) has proven particularly suitable for depositing the auxiliary layer or auxiliary layers as a starting coat by means of cold spraying. In this method, the particles are fed into the divergent part of the convergent-divergent nozzle, and the carrier gas is brought to a comparatively low pressure for cold spraying. In this case, the particle speeds are lower than when the particles are fed into the stagnation chamber positioned upstream of the nozzle and are accelerated by a higher pressure level of the carrier gas, which is the usual practice. In LPGDS, therefore, the mechanical stress on the mold core when the particles impinge upon the surface thereof is also lower. Since the material of the mold core is in any case sensitive to the cold gas jet, the particles nevertheless adhere to the mold core without permanently destroying it.

According to one embodiment, the at least one auxiliary layer is removed from the hollow body after the removal of the mold core from the mold. The removal of the mold core from the mold can be performed by conventional methods from the prior art. A sand core or other lost cores can be melted out or destroyed with the aid of ultrasound, for example. Using plastic or wood or, alternatively, metal, it is possible to produce assembled cores, which can be of multipart design to enable them to be removed as individual parts from the finished hollow body. The auxiliary layer then remains in the cavity formed by the hollow body since, as a result of the mechanical deformations due to the impinging particles of the hollow body material, it is firmly connected to these.

If the material of the auxiliary layer does not interfere with the functioning of the hollow body produced, it can remain within the hollow body as a lining of the cavity. As already indicated, the material of the auxiliary layer can even assume additional functions in the hollow body, such as corrosion protection or an antimicrobial action. However, if the material of the hollow body is supposed to form the inner wall of the cavity, the auxiliary layer must subsequently be removed. This removal can be achieved mechanically, e.g. by sandblasting. One alternative is to remove the material by means of a selective etching method, wherein the etchant attacks the material of the hollow body only a little or not at all.

FIG. 1 shows schematically a cold spraying system, which is accommodated in a process chamber 11. The cold spraying system is reduced to its essential components and thus represents only a schematic diagram. The cold spraying system has a convergent-divergent spray nozzle 12, which is connected to a unit 13 having a stagnation chamber (not shown). A mold core 16 is held by means of an industrial robot 15 in such a way that said mold core can be coated by a cold gas jet produced by means of the spray nozzle 12. This coating process takes place in several stages. Via a first storage container 18, particles of a tin solder are introduced into the divergent part 19 of the spray nozzle 12 and are accelerated in the cold gas jet 17. These form a first auxiliary layer (not shown specifically in FIG. 1) on the mold core 16 (cf. also FIGS. 3 and 4). Copper particles are then introduced from a second storage container into the stagnation chamber (not shown specifically) ahead of the spray nozzle 12 and are likewise accelerated by means of the cold gas jet 17 to the former 16 coated with the first auxiliary layer. A second auxiliary layer is formed, wherein these two auxiliary layers form a starting coat 21 (cf. FIGS. 3 and 4). Finally, titanium particles are taken from a third storage container 22 and are likewise mixed in with the cold gas jet 17 via the stagnation chamber. Here, several layers of titanium can be applied to the starting coat, thereby forming a wall of a desired thickness of a hollow body to be produced, wherein the cavity enclosed by the hollow body is defined by the mold core 16.

FIG. 2 shows a hollow body 23 of the type that could be produced with a cold spraying system shown in FIG. 1. For this hollow body an assembled mold core 16 has been used, comprising a plurality of shaped elements 24 made of wood. Integrated into the shaped elements are joining aids 25, which define the position of the individual shaped elements 24 relative to one another and in this way facilitate assembly. At the same time, these joining aids are embodied in such a way that the mold core can be removed from the cavity of the hollow body 23 without destroying the individual shaped elements 24. The mold core 16 was adhesively bonded to a metal foil 26 before coating with the material of the hollow body 23, although the adhesive coat as such is not shown in FIG. 2. Coating to form the hollow body 23 is then performed by means of cold spraying. The state after the coating has been produced is shown in FIG. 2. After the coating has been produced, the shaped elements 24 can be removed from the cavity of the hollow body 23 in the manner already described, wherein the adhesive bond between the mold core 16 and the metal foil 26 is made weaker than the bond between the hollow body 23 and the metal foil 26 brought about by cold spraying. The metal foil 26 therefore remains in the cavity, while the adhesive bond dissolves. Insofar as required, this can be removed from the cavity in a manner not shown, by a selective etching method, for example.

FIG. 3 shows a mold core of bonded sand. This is coated with the starting coat 21 produced in accordance with FIG. 1, wherein the hollow body 23 composed of titanium was produced in a subsequent step. Not shown in FIG. 3 is the possibility of destroying the sand mold core by means of ultrasound, for example, thus ensuring that the core is lost and can be removed from the cavity of the hollow body 23. The starting coat 21 remains in the cavity, as already described with reference to FIG. 2. Here too, there is the possibility of subsequently removing it mechanically or chemically.

FIG. 4 shows the detail IV in FIG. 3 on an enlarged scale. It becomes apparent that the sand core is first of all coated with a base layer 27 which, according to FIG. 1, is composed of a tin solder. Alternative materials would be a white metal (alloy containing tin) or zinc. This base layer is followed by a top layer 28, which is composed of copper and makes available a surface 29 for coating with the material of the hollow body (in this case titanium). As an alternative to copper, the top layer can also be composed of zinc or aluminum or alloys which contain at least one of these metals. Further auxiliary layers can be produced between the base layer 27 and the top layer 28 in order, for example, to make the transition to various property profiles of the auxiliary layers (ductility, hardness and/or temperature stability) smoother. 

What is claimed is:
 1. A method comprising, in the following order: forming a starting coat on a mold core, wherein the auxiliary layer is metallic or contains predominantly metallic components, coating the mold core using a cold spraying process to produce a hollow body, removing the mold core from the hollow body, and removing the starting coat from the hollow body.
 2. The method of claim 1, wherein the starting coat has a different material composition than the hollow body.
 3. The method of claim 1, wherein the starting coat comprises an aluminum foil.
 4. The method of claim 3, comprising adhesively bonding the aluminum foil onto the mold core.
 5. The method of claim 1, wherein the starting coat is formed on the mold core by low-pressure gas dynamic spraying of a metallic material.
 6. The method of claim 5, wherein the metallic material comprises one or more of zinc, tin, lead, aluminum, copper, silver, or gold, or an alloy of zinc, tin, lead, aluminum, copper, silver, or gold.
 7. The method of claim 6, wherein the starting coat has a multilayer design including multiple auxiliary layers, wherein forming the starting coat comprises forming multiple auxiliary layers of increasingly harder and/or higher-melting metallic materials.
 8. The method of claim 7, wherein the starting coat is produced by forming two auxiliary layers comprising: a base layer lying on the mold core and composed of zinc, tin or lead, or a metal alloy based on zinc, tin, or lead, and a top layer formed over the base layer and composed of zinc, aluminum, copper, silver, or gold, or a metal alloy based on zinc, aluminum, copper, silver, or gold.
 9. The method of claim 1, wherein the mold core is produced from wood, plastic, metal, or bonded sand.
 10. A mold core, comprising: a surface suitable as a substrate for cold spraying, wherein the surface is formed by at least one auxiliary layer composed of a metallic material, which forms a starting coat on the mold core.
 11. The mold core of claim 10, wherein said mold core consists of bonded sand, wood, metal, or plastic. 